The present invention relates to the fabrication of mica sheet and particularly to a method for making thin, flexible mica film by a continuous electrophoresis process. The film is useful for electrical applications or the like, such as for example a solid electrolyte or electronic insulator.
The preparation of mica sheet by electrophoresis is known. For example, H. R. Shell and K. H. Ivey disclose, in Fluorine Micas, U.S. Bur. Mines Bull. 647 (1969) page 223, a batch process wherein delaminated mica crystals about 5-25 microns in diameter are electrophoretically deposited from an n-amyl alcohol suspension onto a nickel or molybdenum cathode. The process produces relatively thick, porous mica films which are subsequently stripped from the metal cathode for recrystallization.
In Ceramic Industry, (May, 1980) page 30, F. Handle describes a process for forming clay sheet for dinnerware or tile manufacture. This apparatus utilizes electrophoretic deposition onto counter-rotating zinc-coated cylinders to produce thick (0.08-0.79 in.) sheet.
It has also been proposed to prepare nonwoven materials from organic components using electrophoresis. The deposition of nylon fibers in a polyacrylate binder onto a shaped anode to form a nonwoven fabric is described by R. D. Bankert, et al. in Textile Res. J, Vol. 43 (1973), page 247. Brass and aluminum anodes were found to be preferred for this deposition process.
It has recently been discovered that polycrystalline mica film or paper can constitute a suitable solid electrolyte for use in electrochromic devices. The copending, commonly assigned application of G. H. Beall et al., Ser. No. 218,937 filed Dec. 22, 1980 discusses the fabrication of electrochromic devices incorporating mica paper or film electrolytes, and suggests that mica layers for this purpose can be directly deposited on a device electrode by electrophoresis. Mica sols containing small, uniformly sized mica crystals produced in accordance with the teachings of U.S. Pat. No. 4,239,519 were used for this process. However, mica deposition in that case was onto a permanent substrate since the device prepared was not self-supporting.
It is therefore a principle object of the present invention to provide an improved method for providing mica sheet in thin film form for applications wherein strong, self-supporting flexible mica film is required.
It is a further object of the invention to provide a technique for fabricating mica film which is particularly suited to the manufacture of a transparent, solid-state mica electrolyte for thin-film electrochromic devices or the like.
Other objects and advantages of the invention will become apparent from the following description thereof.